Method for providing metal coatings



Jan. 2, 1968 ULRICH ETAL 3,361,562

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United States Patent Ofifice 3,361,562 Patented Jan. 2, 1968 3,361,562 METHOD FOR PROVIDING METAL COATINGS Erhard Ulrich and Horst Schreiner, Nuremberg, Germany, assignors to Sicmens-Schuckertwerke Aktiengesellschaft, Berlin and Erlangen, Germany, a corporation of Germany Filed Oct. 19, 1965, Sel'.N0. 497,899 Claims priority, application Germany, Dec. 18, 1964, S 94,677, S 94,678, S 94,679 8 Claims. (Cl. 75-208) ABSTRACT OF THE DISCLOSURE Method for providing a metallic layer on an underlying metal substrate body comprises roughening the surface of the body, isostatically pressing a metal powder layer on the roughened surface, sintering the layer on the body, and then melting the layer in incremental steps to provide a controlled melting of the layer.

Our invention relates to a method for producing metal coatings on underlying substrates. More particularly it relates to such improved method and apparatus for applying such coatings from difierent types of metallic materials.

Many methods are, at present, known for the providing of a metallic layer on an underlying surface of a substrate body. For example, well-known processes for applying a corrosion-resistant metal coating, especially in a galvanic manner include flame-spraying, welding and plating. However, processes for applying metallic coatings galvanically present many problems. One of these problems is the difiiculty of achieving thick metallic layers. Where a welding process is employed, diflicultics result from the restricted accessibility, particularly, for example, in the building up of metallic layers on the inner surfaces of small and irregular-shaped substrate bodies. In addition, where the substrate body is itself of a metallic nature, penetration thereof by the metallic layers applied by welding techniques results in undesired alloy formation. Where a plating technique such as roll plating is employed, only plane or other simple surfaces can be coated. Where a flame spraying method is used, metallic coatings of desired thickness are difiicult to attain and similar to the welding method, proper accessibility of the flame spray apparatus to inner surfaces is diflicultly achieved.

In addition to the foregoing galvanic processes for applying metallic coatings, powder metallurgy methods are also known. In such methods, it is a common practice to carry out a compacting in a mold over at least one ram which is designed and adapted for use as an annular ram. With such known pressing method, the depositing of a metal powder layer is particularly difiicult when the desired thickness of the metallic layer to be formed is thin as compared to the length of the underlying substrate body such as a tube. It is known that through a reduction of the axial pressure applied to the metallic powder, there may be achieved at the compacting of two rams, a coating thickness which has a minimum thickness of 0.1 to 0.05 times the pressing height. Furthermore, where a larger ratio of coating thickness to pressing height exists, a coating is produced which is uneven in density, such unevenness concomitantly resulting in deviations in desired characteristic values.

Accordingly, it is an important object of this invention to provide a method for providing a metallic coating on an underlying substrate body wherein there is enabled the obtaining of varied coating layer thicknesses of even density, on surfaces of varied shapes and degrees of ready accessibility and without the producing of undesired alloying between the metal coating and an underlying metal body substrate.

Another object of this invention is to provide a method for producing sintered coatings from metal powders on a compact metal or sintered body.

A further object is to provide apparatus for carrying out the method set forth in the preceding objects.

In a first embodiment of a method in accordance with the invention in which the foregoing objects are achieved, a metal powder layer is pressed isostatically onto a mechanically or chemically pretreated substrate surface and the powder layer is then consolidated by the sintering thereof. In a particular development of this embodiment, the pores of a sintered metal powder layer can be filled by the impregnating thereof with a liquid metal, such liquid metal being used in an amount sufficient for the complete impregnation of the metal powder layer. In some situations, it may be preferable to first provide the mechanically or chemically pretreated surface with a coating of a metal or a metal alloy and to isostatically press the metal powder layer on the latter coating.

With the method of the foregoing embodiment a uniform and relatively high degree of compacting of the metal powder is achieved. A particular advantage of this method embodiment is that it can be carried out with relatively simple pressing dies and presses and that high pressures can be transmitted to large surface areas. Particularly, this embodiment of the method according to the invention enables the coating of surfaces having complex shapes and the coatings resulting therefrom are especially suited as protective surface coatings for workpieces, tools, and instrument and machine parts which are normally subjected to mechanical wear such as sliding friction, scaling at elevated temperatures, and corrosion.

in this method embodiment, the underlying substrate body whose surface is to be coated is first pretreated mechanically, i.e., essentially it is roughened. Mechanical roughening is readily effected in a known manner by grinding, grooving, sand blasting and the like. Alternatively, the surface can be treated by etching it with acids, by an anodic treatment, or by other known suitable chemical treatments. A coating which in the end product forms an intermediate layer can be applied by electroless or galvanic separation from metal in the form of compact or spongy deposits. In another application according to this method embodiment, the coating is formed by the spreading on of a metal powder or a metal powder mixture in paste form. This method embodiment is not lim ited to any particular metal powders both with reference to the metal powder and to any intermediate layers. For use at relatively elevated temperatures, metal powders which are preferable are those of chromium, nickel, cobalt, molybdenum, tungsten, and their alloys. In addition, powder mixtures and compound powders containing carbon, carbides, oxides, nitrides and borides may be advantageously utilized. The metal powder is suitably isostatically pressed onto a treated or coated surface of an underlying substrate body at a pressure of 1-15 mp./ square cm., preferably 3-6 mp./square cm. Isostatic pressures up to about 4000 atmospheres can be attained in larger pressure tanks, the bodies to be coated being placed in these tanks. For higher compression pressures such as up to 15,000 atmospheres, abs. (15 mp./square cm.), the arrangement described hereinbelow is relatively simple, economical and technically facile to achieve. In this latter arrangement, the pressure is produced in an ordinary press.

In accordance with the method of this embodiment, the coating is preferably subjected to sintering in a reducing atmosphere such as a current of hydrogen or in a vacuum. The sintering temperature and sintering period are so chosen whereby a maximum compacting takes place with the resulting layer of metal powder still being present in a porous state. A sintering temperature at which the metal powder forms a liquid phase has proven to be particularly suitable. Thus, using an alloy powder having a composition of 67.0 percent of chromium, 4.6 percent tungsten and 1.4 percent carbon in admixture with a 2 percent graphite powder, there is obtained, after an isostatic pressing at 3t/ square cm. on a compact ring of steel, such as that known as steel 37, and thereafter sintering for one hour at l200 C. in an atmosphere of forming gas, in consequence of the liquid phase, an extensive compact sintering of the metal powder layer on the compact ring is effected.

In accordance with this embodiment of the inventive method, a liquid phase can also be formed between the metal powder and the compact body. In some situations it may be sufficient for one component of the metal powder or of the substrate body to form a liquid phase with adjacent layers. When an intermediate layer is used, the liquid phase may be formed between the intermediate layer and the metal powder layer, or betwen the intermediate layer and the underlying substrate body, or respectively between both the metal powder and the intermediate layer, and the intermediate layer and the underlying body. The pores can then be filled by the impregnation thereof with a liquid metal. The latter impregnating metal may suitably be an alloy of copper, silver, chromium, cobalt, nickel, manganese and silicon, as well as high-temperature solders.

In accordance with a preferred application of the embodiment according to the invention the metal powder, at the isostatic pressing step, is filled, in dry or wet form, into a hollow space of chosen configuration between the substrate body and a plastic container of predetermined shape and is then compacted in a mold with two press rams over an elastic receptacle and a plastic material.

The sintering and impregnation of the porous sintered layer can be carried out in a single operational step. The required amount of impregnating material is thereat already brought into contact with the sintering structure during the sintering. The temperature is chosen such that the metal still remains solid during the sintering. Only after the sintering is completed is the temperature raised to the point where the metal becomes liquid and is drawn into the pores of the structure.

In a second embodiment of the method according to the invention, a metal powder is uniformly pressed by isostatic pressing on the mechanically or chemically pretreated surface of a compact or porous underlying substrate body. Advantageously, the body first has provided thereon, a coating of a metal or a metal alloy. Thereafter, the layer of metal powder is, according to the invention, melted down to form a non-porous coating. In some applications, it has been found advantageous to consolidate the metal powder layer by sintering prior to the aforementioned melting down. The sintering of the isostatically pressed on metal powder layer can be directly followed by a mechanical treatment, and should be free of scale, rust and grease. The surface can suitably also be subjected to a chemical treatment, such as the etching thereof with acids. The adhesion of the metal powder layer can be improved if the surface of the substrate body is subjected to an anodic treatment. On the pretreated surface of the substrate body, the layer of metal powder is isostatically pressed, and this layer can then be melted down after the sintering treatment if such melting down is required. In some applications of the method of this embodiment, a coating is applied to the treated surface of the substrate body. Suitable materials for producing this coating on the substrate body, whether the latter is compact or porous are, for example, cobalt or nickel, or an alloy of nickel, cobalt, or chromium. This metallic intermediate layer can be deposited on the substrate body by an electroless or a galvanic technique.

In some situations, in this method embodiment, the

spreading of the metal powder or metal powder mixtures in the form of a paste has proven to be advantageous. With a suitable composition for the coating, a diffusioninhibiting effect can be achieved and concentration dislocations in the metal powder layer and the substrate body can be reduced. Consequently, different coefficients of expansion of the melted down powder layer and of the substrate body can thereby largely be bridged.

The isostatic pressing on of the metal powder deposited on the intermediate layer may suitably be effected at a pressure of 3 to 6 mp./square cm. Suitable metal powders may be those of such metals, for example, as cobalt, chromium, iron, nickel, titanium or silicon as well as mixtures and alloys of these metals with such metals as tungsten molybdenum, niobium and tantalum. Carbides, oxides, borides, nitrides, and silicides are suitable as occlusion components.

The isostatic pressing is suitably generally carried out in pressure tanks filled with a liquid. In a particular application of this embodiment, the metal powder is filled into a hollow space at the isostatic pressing step, either in the dry or wet state, between the substrate body and a plastic receptacle and compressed using at least one press ram over an elastic container and a plastic material.

In a third embodiment of the method according to the invention, an elastic filling compounds is introduced into a jacket and the metal powder to be compacted is filled in between the filling compound and the jacket and is then compacted by at least one ram over the elastic compound. Thereafter, elastic material is released and removed and the metal powder which has been compacted on the jacket is consolidated by the sintering thereof. The particular advantage presented by this embodiment lies in the fact that the metal powder is compacted directly inside the jacket and the compacted metal powder remains therein. The jacket itself serves as the mold. Thus, for example, a cobalt-chromium-tungsten powder can be compacted directly in a tube, a sleeve or a profile body and remain therein, the compaction being effected isostatically. With this technique, the use of a relatively small force can effect a substantially uniform distribution of pressure over a relatively large cylindrical surface. Preferably, the jacket is prestressed to a point close to the elastic limit (yield point) when the metal powder is pressed in. Consequently, upon the releasing of the pressed in ring of powder, it is thereby put under a pressure prestress. Thereafter, at the subsequent sintering, the shrinkage resulting from the sintering is, in part, compensated for thereby. As a result, there is obtained a sintered coating which exhibits quite good adhesion to the jacket.

In accordance with the invention, the transmission of the pressure is effected by means of an elastic material. An elastic material which is suitable for this purpose, for example, may be rubber, a thermoplastic material, a silicone rubber, and the like. The elastic material can be employed in the form of a shaped member such as a cylinder, for example. It is particularly advantageous to use the elastic material in the form of a hose which is filled with a liquid or with a paste. In the latter instance, filling materials of low compressibility are preferable.

According to a particular feature of this embodiment, the jacket can be further surrounded by a supporting structure, such structure being required in a case where the strength of the jacket might not be adequate at the necessary compression pressure and where, consequently, the danger exists that the jacket might be deformed. In this embodiment of the inventive method, there may suitably be used all types of metal powders, metal powder mixtures, alloy powders, and compound metal powders.

Generally speaking and in accordance with the invention, there is provided a method for providing a metallic coating on an underlying substrate body which comprises the steps of isostatically pressing a metal powder on the surface of the substrate body and thereafter sintering the metal coated body.

Also, in accordance with the invention, there is provided apparatus for providing a metallic coating on an underlying substrate body comprising a jacket, a deformable elastic material container snugly received within the jacket, the container being adopted to receive therein the substrate body and a metal powder, and a pair of rams respectively adopted to be received within the jacket and. to bear with pressure against the opposite ends of the container having therewithin the substrate body and the metal powder.

The foregoing and more specific features of our invention will be apparent from and will be mentioned in the following description of a method and apparatus for providing metal coatings according to the invention shown by way of example in the accompanying drawing.

In the drawing, FIGS. 1 to 3 are cross-sectional views of embodiments of apparatus for carrying out one illustrative embodiment of the method according to the invention with respectively differently shaped substrate bodies;

FIGS. 4 to 7 are crss-sectional views which show further embodiments of apparatus for carrying out the aforesaid one illustrative embodiment of the method;

FIGS. 8 to 10 are views, partly in cross-section, respectively illustrating an apparatus for carrying out another embodiment of the method according to the invention and showing different stages of this other embodiment; and

FIGS. 11 and 12 are views partly in cross-section of further examples of suitable apparatus for carrying out the aforesaid other embodiment of the method.

Referring now to FIGS. 1 to 3 wherein there are respectively shown cross-sectional views of the arrangements of parts to be coated as they are inserted into pressure tanks in accordance with the first embodiment, FIG. 1 shows a parts arrangement for a plate, FIG. 2 shows a parts arrangement for a bolt and FIG. 3 shows a parts arrangement for a sleeve. In FIGS. 1 to 3, the numeral 1 respectively designates the substrate bodies to be coated, the numeral 2 respectively designates the metal powders to be pressed, the numeral 3 respectively designates elastically deformable plastic containers, and the numeral 4 respectively designates elastic sealing parts.

FIGS. 4 to 7 respectively show in accordance with the first embodiment a plate mold, a rod mold wherein the rod has a circular or polygonal cross section, a sleeve mold, and a profile body mold. In these figures, the numeral 5 respectively designates the molds, the numeral 6 designates the bodies to be coated, the numeral 7 respectively designates the elastically deformable plastic containers for the isostatic pressing, anud the numeral 8 designates the respective metal powders. In FIGS. 4, 6, and 7, the numeral 9 respectively designates the cylindrical upper rams and the numeral 10 respectively designates the cylindrical lower rams. In FIG. 5, the annular upper ram is designated by the numeral 11 and the annular lower ram is designated by the numeral 12. In the embodiments according to FIGS. 6 and 7, viz., the sleeve and profile body molds, the tanks therein, respectively designated by the numeral 13, are filled with either a plastic material or a liquid 14. In all of the embodiments of FIGS. 4 to 7 respectively, the substrate body is ejected from mold 5. In a variation of the arrangement, the substrate body 6 can serve directly as the mold if it has an adequate thickness in which case, mold 5 may be dispensed with. In this latter variation, the dimensions of rams 9 and 10 must then, accordingly, be conformed to the inner diameter of the substrate body.

According to the first embodiment, workpieces can be coated on their outsides. This embodiment of the inventive method also lends itself particularly well to the coating of workpieces on their insides, particularly sleeves whose length to diameter ratio is greater than two and to the coating of small parts which cannot be coated by the hereinabove set forth known methods.

According to the second embodiment and with reference to FIGS. 5 and 6, the melting of the pressed powder layer can take place after the isostatic pressing and even after the sintering step. Heat may be supplied by an oxyacetylene, arc, or plasma flame, or particularly advantageously with inductive heating. It is important that the melting down be effected with measured heat portions in order that alloying between the metal powder layer and the substrate body and any existing intermediate layer may be controlled. In the melting down, there is first suitably produced a predetermined melt depth in the layer. The range of the melt is then allowed to spread over the entire surface.

Solid substrate bodies coated in accordance with this second embodiment of the inventive method are characterized by a high degree of hardness, mechanical resistance, resistance to heat, and corrosion resistance. The melting process can readily be made automatic and mechan-ically carried out whereby an accurate monitoring of the amount of melted down material and, consequently, of layer thickness is thereby enabled. With this embodiment, those areas which are difiicultly accessible in welding processes can readily be properly coated. A particular advantage presented by this embodiment is that it enables the coating of objects both on their inside and their outside surfaces. Thus, for example, on bolts and long bushings, uniform compact coatings of metal can be deposited, an achievement not attainable with known methods.

Referring now to FIG. 8 wherein there is shown a View partly in cross section of a device for carrying out the third embodiment of the method according to the invention, the structures designated by the numerals 2 and 3 are the upper and lower rams respectively. Numeral 4 designates the elastic filling compound, the structure 1 being the jacket. The metal powder 5 is contain d in the annular space defined by filling compound 4 and jacket 1.

FIG. 9 shows the conditions obtained in the device of FIG. 8 during the pressing step. In such step, rams 2 and 3 may suitably be compressed in a press of 70!, for example at a ram surface of about 12.6 square cm., such quantities corresponding to a specific ram pressure of 5.6t/square cm. The metal powder is thereat compacted on the inside of jacket 1 over elastic material 4.

After the pressure is removed from rams 2 and 3, elastic material 4 can be easily removed. The metal powder is compacted on the inner surface of jacket 1 and forms a firmly adhering ring or hollow cylinder therein. The internally coated jacket 1 can now be sintered and, if it is so required, subject to a further treatment after such sintering.

In FIG. 11, there is shown another example of a device for carrying out this third method embodiment. On the depiction shown in FIG. 11, the elastic inner material comprises an elastic material hose 6. The metal powder 5 is contained in the annular space defined by the inner surface of jacket 1 and the outer cylindrical surface of hose 6. Elastic material hose 6 is filled with a liquid or a paste and then closed by means of a pair of plugs 7 and 8. The pressing step then proceeds as shown in FIGS. 9 and 10 to result in a pressed final product as shown in FIG. 10.

FIG. 12 shows a further example of a device for carrying out this third embodiment. In FIG. 12, numeral 1 designates an annular jacket and numeral 2 designates the upper ram. The numeral 10 designates a support structure which suitably comprises a two part cylinder with extending flanges which are held together by clamps 11.

It will be obvious to those skilled in the art upon studying this disclosure that methods and apparatus for providing metal coatings according to our invention permit of a great variety of modifications and hence can be given embodiments other than those particularly illustrated and describe-d herein without departing from the essential features of our invention and within the scope of the claims annexed hereto.

We claim:

1. A method for providing a metallic layer on an un- 7 derlying metal substrate body comprising roughening the surface of said body, isostatically pressing a metal powder layer on said roughened surface, sintering said layer on said body, and then melting said layer in incremental steps to provide a controlled melting of said layer.

2. A method for providing a metallic layer on an underlying metal substrate body comprising roughening the surface of said body, depositing a metal coating on said roughened surface, isostatically pressing a metal powder layer on said roughened surface, sintering said layer on said coated body, and then melting said layer in incremental steps to provide a controlled melting of said layer.

3. A method for providing a metallic layer on the inner surface of a metallic jacket comprising introducing an elastic filling material into said jacket in a configuration to define a hollow annular space between the outer surface of said filling material and the inner surface of said jacket, filling said annular space with a metal powder, isostatically pressing said powder to compact it in a layer on said inner surface, removing said plastic material, sintering said metal layer on said inner surface of said jacket.

4. A method for providing a metallic layer as defined in claim 3 wherein said jacket is prestressed to a point close to its elastic limit when said powder is pressed.

5. A method for providing a metallic layer as defined in claim 3 wherein there is utilized a support structure which surrounds said jacket.

6. A method as defined in claim '3 wherein said elastic filling material is in the form of a hose which is filled with a liquid.

7. A method as defined in claim 3 wherein said elastic filling material is in the form of a hose which is filled with a paste.

8. A method as defined in claim 3 wherein said metal powder is isostatically compacted at a pressure of between 1 and 6t/square em. over a cylindrical surface having a length 5 to 10 times the cylindrical diameter.

References Cited UNITED STATES PATENTS 2,187,086 1/1940 Koehring 75-208 X 2,190,237 2/1940 Koehring.

2,198,240 4/1940 Boegehold 75-208 X 2,198,253 4/1940 Koehring 75-208 X 2,198,254 4/1940 Koehring 75-208 X 2,251,410 8/1941 Koehring 75-208 X 2,902,748 9/1959 Schaefer 75--208 X 2,986,464 5/1961 Lewis 75--208 3,006,069 10/1961 Rhoads.

3,154,844 11/1964 Sayre 75-208 X L. DEWAYNE RUTLEDGE, Primary Examiner. BENJAMIN R. PADGETT, Examiner.

A. J. STEINER, Assistant Examiner. 

